Hmmm... this is a bigger question than it seems. Firstly, I'll categorise between OS's then by types of protection.
Unfortunately for Windows users (ie the vast majority out there) pretty much most viruses are targeted for this platform due to the mere fact that it has 95% of the desktop market. Too bad but here is a breakdown of the different types of threats with some solutions.
1.1 Firewall. Windows comes with its own firewall but is generally not considered very full featured. I think you can't go past ZoneAlarm as a firewall and it has a free version which has enough features for home use.
1.2 Anti-virus. I did hear that Windows was going to provide a free anti-virus package but I don't know if this is available as of Windows 7. In any case, even if they did, it would likely not be as good as freely available software. Avast is a free anti-virus product that seems to rate well in reviews.
1.3 Spyware, adware, malware, etc. This is the newest area of concern for Windows users and has no protection from Windows out of the box. Spyware collects information about you that you have on your computer and sends it back to a central point. Adware are applications that send advertisements to you without you agreeing or wanting them. Malware is basically software that acts in a manner that is detrimental to a users computer. In any case there is excellent software freely downloadable called Ad-Aware from Lavasoft which does an excellent job of catching these threats.
1.4 Hijacking. This is the worst form of threat that I have ever come across. I had my web browser hijacked once and basically whenever you open up your browser, it defaults to going to an insidious website such as a pornographic site or even worse a website which will download all sorts of other viruses. What was so bad about this is that I could not figure out how to fix it. In the end I just had to reinstall windows and be more careful with the software that I installed on my computer and the sorts of websites that I visited. Just hope you never get one of these.
Don't you love the Mac with its funky interface and so easy to use applications? Did you also know that it is based on BSD a Unix-like derivative? You didn't - well it is and that is what makes it so stable, reliable and secure. But I'm not recommending it. Why? Well, I used to tell people that if they wanted the best user interface ever, then they can't go past the Mac. I used to praise its usability and visually stunning front end. That was until my daughters bought iPod Touch's. From that moment on I realised that it's not the viruses that Mac users need to fear, it's the Apple organisation itself! I have never come across a more insidious organisation that digs its fingers into you and never wants to let you go. I could not set up an account for my daughters to download the FREE iPod software without giving Apple my credit card details. I could not connect them up without downloading and installing iTunes. I could not do anything without getting Apple involved somewhere in the process. It was truly Big Brother scary. I no longer recommend Apple as a solution to anyone even though it still has the best operating system and the most usable software. The personal cost is too great.
3. Linux (specifically Ubuntu at the moment)
Wow! This operating system is like a breath of fresh air compared to the alternatives! I use it on my desktop, my server and my netbook and it just works out of the box. I never install any virus protection software and have never had a problem with viruses. This is a great alternative for those that aren't afraid to install their own operating systems (and in fact has a better installation system than Windows). Graphically it can seem quite bland out of the box and it is - out of the box - but customise it even just a little bit and you can do things with it that even Mac and Windows can't. Check out some of the cool 3d effects you can have on your desktop: http://www.youtube.com/watch?v=ZxfSwzhSn1c
but there are lots of other examples. So for an operating system that is FREE, has a great graphical user interface and is VIRUS-FREE then you can't go past Ubuntu.
I'm guessing that section 1 will be of most use to you but I'm hoping that one day you might consider option 3 - especially if price is a factor for you.